Touch panels have been widely used in various applications. A touch panel comprises sensing lines arranged in a crisscross pattern. A sensing circuit is provided for sensing a position of the touch panel where a finger touches.
FIG. 1 is an illustration of a sensing circuit 10 of a touch panel (not shown). The sensing circuit 10 comprises a sensing electrode 100 and a discharging unit 110. The sensing electrode 100 has an intrinsic capacitance, which is a parasitic capacitance represented by a capacitor 104. A total capacitance of the sensing electrode 100 increases when a touch event occurs. The touch event is equivalent to an additional capacitance, which is represented by a sensing capacitor 102. That is, the sensing capacitor 102 has a nonzero capacitance when the touch event occurs, while the capacitance of the sensing capacitor 102 is zero in a condition that no touch event occurs. The capacitance of the sensing capacitor 102 is much less than that of the parasitic capacitor 104 even when a touch event occurs. A modifying capacitor 116 is used to compensate the parasitic capacitance of the sensing electrode 100 so that the parasitic capacitances of the respective sensing electrodes 100 will not significantly differ from each other.
The discharging unit 110 is connected to a voltage source VDD via a switch 114 so as to precharge the capacitances (i.e. the sensing capacitor 102 and the parasitic capacitor 104) of the sensing electrode 100 to a predetermined voltage level such as 5V. A first current source 127 and a second current source 138 discharge in succession by controlling switches 123 and 134. The first current source 127 is a large current source, of which a current level is about 300 μA, for example. The second current source 138 is a small current source, of which a current level is about 90 nA, for example.
To discharge the sensing circuit 10, the first current source (i.e. the large current source) 127 is switched on first to discharge for a predetermined duration. The precharged voltage level of 5V decreases to about 1V. During the duration that the large current source 127 discharges, the second current source (i.e. the small current source) 138 is disconnected. Then the large current source 127 is disconnected, the small current source 138 is switched on to discharge the residual charges. The duration to complete the discharge of the sensing circuit 10 for a condition that a finger touch event occurs is very different from that for a condition that no finger touch event occurs, since the total capacitances are different. Therefore, the duration to complete the discharge can be used to determine whether a touch event occurs or not.
In the discharging duration, the precharged voltage is reduced linearly. The sensing circuit 10 has a comparator 145 for comparing the precharged voltage and a reference voltage Vref with a level of 1V, for example. When the precharged voltage drops below the reference voltage Vref, the comparator 145 transmits a status changing signal. The duration is measured from starting the discharge to transmitting the status changing signal. The sensing capacitance representing the touch event can be calibrated according to this measured duration.
FIG. 2 shows a condition that a finger touch event occurs and a condition that no finger touch event occurs for the same sensing circuit. A first curve is drawn by a solid line which represents the condition that a touch event occurs. A second curve is drawn by a dashed line which represents the condition that no touch event occurs. The total capacitance of the sensing circuit 10 is precharged to the predetermined voltage (e.g. 5V). Therefore, the voltage drop ΔV to be discharged is 5V. Then the large current source 127 discharges from time 0 to time t1. During the large current source 127 discharges, the released voltage drop is ΔV1 in the condition that a touch event occurs, while the released voltage drop is ΔV1′ in the condition that no touch event occurs. As mentioned, the total capacitance of the condition that the touch event occurs is greater than that of condition that no touch event occurs. Then the residual charges are released by the small current source 138. For the condition that the touch event occurs, the corresponding voltage drop of the residual charges is ΔV2, and the discharge is completed at time t2. For the condition that no touch event occurs, the corresponding voltage drop of the residual charges is ΔV2′, and the discharge is completed at time t2′.
Due to the process variance and so on, the parasitic capacitances of the respective sensing electrodes 100 of the touch panel may be different from each other. Although the differences are being compensated by the modifying capacitors 116, the difference cannot be completely eliminated. To correct the residual sight differences, the discharging condition of the large current source 127 needs to be fine tuned. Conventionally, such a fine tuning is carried out by adjusting the current of the large current source 127. In general, the current sources are implemented by CMOS transistors. The current level of the current source is adjusted by controlling a voltage applied to the CMOS transistor. To fine tune the current level, an exact voltage level must be provided, such as 1.9V, 2.0V, 2.1V and the like. However, it is difficult and troublesome to fine tune the voltage level in practice.